Conventional gas turbine engines are enclosed in an engine case and include a compressor, a combustor, and a turbine. An annular flow path extends axially through the sections of the engine. As is well known in the art, the compressor includes alternating rows of stationary airfoils (vanes) and rotating airfoils (blades) that apply force to compress the incoming working medium. A portion of the compressed working medium enters the combustor where it is mixed with fuel and burned therein. The products of combustion or hot gases then flow through the turbine. The turbine includes alternating rows of stationary vanes and rotating blades that extend radially across the annular flow path and expand the hot gases to extract force therefrom. A portion of the extracted energy is used to drive the compressor.
Each airfoil includes a low pressure side (suction side) and a high pressure side (pressure side) extending radially from a root to a tip of the airfoil. To optimize efficiency, the annular flow path for the working medium is defined by an outer shroud and an inner shroud. The inner shroud is typically formed by a plurality of platforms that are integral to the airfoils and that mate with each other. The outer shroud is typically the engine case disposed radially outward of the outer tips of the rotating blades. A tip clearance is defined between the engine case and the tips of the rotating blades.
One of the major goals in gas turbine engine fabrication is to optimize efficiency of the compressor and the turbine so that work is not lost. Although 100% efficiency is ideal, current turbines and compressors operate at approximately 85-90% efficiency, thus loosing approximately 10-15% in potential work. For both the turbines and the compressors, approximately 20-30% of the lost work, or 2-5% of the total efficiency is lost due to tip leakage losses.
Tip leakage occurs when higher pressure air from the pressure side of the rotor blade leaks to the lower pressure suction side of the blade through the tip clearance. The tip leakage reduces efficiency in two ways. First, the work is lost when the higher pressure gas escapes through the tip clearance without being operated on in the intended manner by the blade, i.e. for compressors the leakage flow is not adequately compressed and for the turbines the leakage is not adequately expanded. Second, the leakage flow from the pressure side produces interference with the suction side flow. The interference results from the leakage flow being misoriented with respect to the suction side flow. The difference in the orientation and velocity of the two flows results in a mixing loss as the two flows merge and eventually become uniform. Both types of losses contribute to reduction in efficiency.
During the operational life of the gas turbine engine, the problem of the tip leakage worsens because the tip clearance between the blade tip and the engine case increases with time and thereby allows more flow to leak therethrough. The tip clearance increases primarily because of two reasons. First, during transient operation of the gas turbine engine the blade tips can grind into the stationary engine case. Second, din particles contained in the large volumes of air that pass over the blades are centrifuged towards the rotating blade tips and cause considerable erosion of the tips. In both situations, the tip clearance increases permanently, thereby resulting in greater tip leakage and greater efficiency losses.
The problem of tip leakage has been investigated for many years and no effective and practical solution has been found other than reducing the tip clearances. Most current solutions involve active changing of the tip clearance by adjusting the diameter of the engine case liner. However, the active control of the tip clearance requires additional hardware that adds complexity and undesirable weight to the engine. Thus, there is a great need to reduce tip leakage in gas turbine engines without including a significant weight and cost penalties.